1. Industrial Field of the Invention
The present invention relates to a process for fabricating an insulated gate-structured semiconductor device such as a thin film transistor (TFT) or a thin film diode (TFD), comprising a non-single crystal silicon film formed on an insulating substrate such as a glass substrate or on an insulating film formed on various type of substrate. The present invention also relates to a process for fabricating a thin film integrated circuit (IC) to which TFT or TFD is applied, and more particularly, to a thin film integrated circuit (IC) for an active-matrix type liquid crystal displaying unit.
2. Prior Art
Semiconductor devices developed heretofore comprising TFTs on an insulating substrate (such as a glass substrate) include an active matrix-addressed liquid crystal display device whose pixels are driven by TFTs, an image sensor, or a three-dimensional integrated circuit.
The TFTs utilized in those devices generally employ a thin film nonsingle crystal silicon semiconductor. The thin film non-single crystal asemiconductors can be roughly classified into two; one is a type comprising amorphous silicon semiconductor (a-Si), and the other is a type comprising crystalline silicon semiconductors. Amorphous silicon semiconductors are most prevailing, because they can be fabricated relatively easily by a vapor phase process at a low temperature, and because they can be readily obtained by mass production. The physical properties thereof, such as electric conductivity, however, are still inferior to those of a crystalline silicon semiconductor. Thus, to implement devices operating at an even higher speed, it has been keenly demanded to establish a process for fabricating TFTs comprising crystalline silicon semiconductors. Known crystalline semiconductors suitable for the purpose like this include polycrystalline silicon, microcrystalline silicon, amorphous silicon partly comprising crystalline components, and semiamorphous silicon which exhibits an intermediate state between crystalline silicon and amorphous silicon.
Known process for fabricating crystalline thin film silicon semiconductors includes depositing an amorphous semiconductor film by plasma CVD or low pressure CVD, and applying thereto thermal energy for a long duration of time (i.e., thermal annealing) for crystallization.
In general, silicon semiconductors need to be heated to a temperature of 600xc2x0 C. or higher. More preferably, heating at 640xc2x0 C. or higher is necessary to further enhance the crystal growth. However, such a high temperature heating has a problem of thermally influencing the substrate. Furthermore, since the heating time required for crystallization was several tens hours or longer, productivity was low. Therefore, it has been demanded to lower the heating temperature and shorten the heating time.
As a means to overcome the aforementioned problems, a process for crystallizing the film by increasing the surface temperature of the film to substantially 800xc2x0 C. or higher has been developed. The process comprises irradiating an intense light such as an infrared radiation or a visible light for a duration of about 10 to 1,000 seconds to the surface of the film. This process, which is called as lamp annealing or rapid thermal annealing (RTA), is expected to be process for reducing the influence on substrates, because the duration of heating can be extremely shortened.
However, since the film formed by plasma CVD and low pressure CVD contains a lot of hydrogen combined with silicon, the decomposition reaction of hydrogen is mainly caused by RTA owing to the short time of RTA, that is, the crystallization does not sufficiently proceed. Furthermore, there is a problem that hydrogen is elected to the exterior of the film by the decomposition reaction of hydrogen to degrade the morphology of the film surface. The present invention has been accomplished in the light of the above circumstances. Accordingly, an object of the present invention is to provide a silicon film suitable for forming a semiconductor device and having a sufficiently high crystallinity.
The process according to the present invention comprises a first step of forming a non-single crystal semiconductor film on a glass substrate and crystallizing the non-single crystal semiconductor film by a thermal annealing and the like to eject hydrogen from the non-single crystal semiconductor film and a second step of heating the non-single crystal semiconductor film by irradiating an intense light thereto (RTA process). Another step of forming, on the surface of said silicon film, an insulating coating which absorbs less than 10% of the intense light used in the second step may be incorporated between the first and the second steps. It is preferred in the present invention that the silicon film obtained by the first step has a low degree of crystallinity, more specifically the degree of crystallinity is 1 to 50%, more preferably 1 to 10%. The first step can be carried out by thermal annealing or other crystallizing methods.
There may be provided a step of patterning the silicon film into at least one island by etching.
As a substrate of this invention, it is preferable to utilize a glass substrate with strain point from 550xc2x0 C. to 680xc2x0 C. Specifically, No. 7059 of Corning Co. (strain point 593xc2x0 C.), No. 1733 of the same (strain point 640xc2x0 C.), LE30 of HOYA Co. (strain point 625xc2x0 C.), NA35 (strain point 650xc2x0 C.) of NH Technoglass Co., NA 45 (strain point 610xc2x0 C.) of NH Technoglass Co., E-8 of OHARA Co. (strain point 643xc2x0 C.), OA-2 of Nihon Denki Glass Co. (strain point 625xc2x0 C.), AN1 (strain point 625xc2x0 C.) of Asahi Glass Co., AN2 (strain point 625xc2x0 C.) of Asahi Glass Co. and the like are desirable. However, a glass substrate other than above mentioned can be utilized, too.
Moreover, an insulating film such as silicon oxide, silicon nitride, or aluminum nitride can be formed on the surface of the glass substrate and an amorphous silicon film can be formed on it. In the case that a film of a material with high heat conductivity like aluminum nitride is formed on a glass substrate, the second step above mentioned can be omitted.
In case that the crystallizing in the first step above mentioned is carried out by thermal annealing, temperature and time of the thermal annealing is varied according to the thickness, composition and the like of the semiconductor thin film. In the case of substantially intrinsic silicon semiconductor, 520 to 620xc2x0 C., for example, 550 to 600xc2x0 C., and 1 to 4 hours is appropriate. It is preferable if the thermal annealing is performed at a temperature lower than the strain point of the glass substrate.
In addition, it is especially preferable if silicon ions have been implanted in the silicon film by an ion implanting method at a dose of 1xc3x971014 to 1xc3x971016 cmxe2x88x922 before the above-mentioned thermal annealing, because crystal growth by thermal annealing is suppressed.
Metal element promoting crystallization of the silicon film, e.g. nickel and the like, can be included in the silicon film obtained by the above-mentioned first step of the present invention. As a metal element other than nickel which can promote crystallization like this, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Sc, Ti, V, Cr, Mn, Cu, Zn, Au, and Ag is known. In an amorphous silicon film added with these elements, crystallization proceeds enough even by low temperature short time thermal annealing of 520 to 620xc2x0 C. and 1 to 4 hours. Crystal growth by RTA later has no effect if the crystallization proceeds excessively. Therefore, in the case of adding these metal elements, it is desirable if time of thermal annealing is shorter, or temperature of thermal annealing is lower, than that of the substantially intrinsic silicon film.
If these metal elements are included in the silicon film, it is also possible to crystallize the silicon film at a lower temperature in the process of RTA later. These metal elements also have an effect of promoting elimination (ejection) of hydrogen from the amorphous silicon film during thermal annealing. If silicon ions have been implanted to the silicon film as above mentioned before thermal annealing process, crystallization growth during thermal annealing can be suppressed, which is preferable, even if these metal elements are added.
These metal elements have great effects on semiconductor characteristic and reliability. A silicon film crystallized with including these metal elements at a large amount especially lacks in reliability durability in the long run. To solve a problem like this, concentration of these metal elements existing in the Si film should be made effective in performing crystallization at a temperature needed, and will not take bad effects on the semiconductor character. Specifically, it is preferable if the minimum value of detection concentration is 1xc3x971015-1xc3x971019 cmxe2x88x923 when concentration of these metal elements included in the silicon film is analyzed through the depth of the silicon film by secondary ion mass spectrometry (SIMS).
As a light to be utilized for RTA in the second step above mentioned, it is preferable if wavelength of the light to be utilized is absorbed in the silicon film but is not substantially absorbed to the glass substrate. That is, it is preferable if the center wavelength lies in the near infrared radiation or visible light. For example, a light with wavelength of 4 xcexcm to 0.6 xcexcm is desirable (e.g. infrared light having a peak at wavelength 1.3 xcexcm). By irradiating intense light like this for relatively a short time like 10 to 1000 seconds, the silicon film can be heated and crystal character can be improved. It is desirable if the silicon film is heated at 800 to 1300xc2x0 C.
If the silicon film is suddenly heated from a room temperature to a high temperature like this, or on the contrary if the silicon film is suddenly cooled from a high temperature like this to a room temperature, effect of stress and the like to be taken on the silicon film is big. Therefore preheat process to heat the film at a lower temperature than this high temperature, or post heat process to heat the silicon film at a temperature between this high temperature and the room temperature for a while in the course of descending temperature from the high temperature condition can be provided. To prevent heat damage on the substrate, it is preferable if temperature of the preheat process and the postheat process is lower than the strain point of the glass substrate by 50 to 200xc2x0 C.
By thermal annealing in the first process, a nucleus of crystal growth is it at least generated, and a low crystallized silicon film (crystallized area is 1 to 50%, preferably 1 to 10% (the rest is amorphous condition)) can be obtained even if the crystallization is suppressed. However, it is not preferred that a semiconductor device is formed by directly using the crystalline silicon film obtained by the first step. This is because there are left a lot of amorphous components mainly in the grain boundary and characteristic of the silicon film is not preferable in bulk and surface thereof. In the present invention, this silicon film is converted to a silicon film with good crystal character by RTA of the second process. By RTA, the silicon film is heated, and crystallinity of the crystallized silicon film is improved and the film is densified simultaneously. In this time, in case that the silicon film has a low crystallinity degree, crystallization can be expanded from the crystal nuclei formed by the first step to the surrounding amorphous region. In this case, since the crystallization proceeds relatively long distance, the effect of decreasing grain boundaries is obtained. By improving. the crystallinity in this manner, the high quality silicon film with 90% or more thereof in area being crystallized can be obtained, which can be utilized for a thin film transistor (TFT).
However, in such RTA, temperature partially changes rapidly. Thus because of difference in thermal expansion coefficient between the silicon film and the substrate, and difference of temperature between the surface of the silicon film and the interface of the substrate and the silicon film, the silicon film is often peeled off. This is particularly notable in the case that the area of the film is so big that it covers the whole surface of the substrate. Therefore, peeling and the like of the film can be prevented by dividing the film in sufficiently small areas, and by making distances between the divided films sufficiently wide so that unnecessary heat would not be absorbed by the substrata. In this way, because the whole surface of the substrate will not be heated through the silicon film, thermal shrinkage of the substrate is suppressed mostly.
In the present invention, it is preferred that light with wavelength of 0.6 to 4 xcexcm is irradiated to the silicon film formed on the glass substrate in the second step (RTA). This wavelength is efficiently absorbed to a low crystallized intrinsic or substantially intrinsic (concentration of phosphorous or boron is 1017 cmxe2x88x923 or less) silicon film, and is converted to heat. Far infrared light with wavelength of 10 xcexcm or more is absorbed to the glass substrate and heats the glass substrate. However, in the case that wavelength of far infrared light is mostly 4 xcexcm or less, absorption by the glass is 0.01 to 10%. Thus heating of the glass is extremely small. That is, to crystallize a silicon film with low crystal character and not to take great effect on the substrate, wavelength of 0.6 to 4 xcexcm is appropriate.